Following the events of Breaking Point, Twilight Sparkle and her friends discover that Equestria's demise may not come from an enemy invasion, but from within. The Barrier Lands and the Heartland begin the slow and uneasy process of reuniting into a single Equestria, but this long-delayed joining is already in danger at its conception.
With Celestia's power in her grasp and her agents by her side, the wheels of Black Rose's plans for Equestria's future continue to turn. However, her brazen gamble has done more than just bring down a centuries-old barrier. Ancient powers begin to stir: the remnants of a dark period in Equestria's history, the immortal inhabitants of Celestia's homeworld, and a rising tide of darkness coming ever closer.